1. Technical Field
The present invention relates to a cast product having a ceramic insert such as a cylinder head, a piston and an exhaust manifold which product is repeatedly subjected to high thermal stress and a method of making such a product.
2. Background Art
Generally, a cylinder head and a cylinder liner which define a combustion chamber of an internal combustion engine are made from casting iron, respectively. Therefore, the combustion chamber is always exposed to possible breakage due to poor rigidity which is a characteristic of the casting iron, and due to residual stress from casting of the cylinder head and the cylinder liner as well as thermal stress and thermal shock exerted on the cylinder head and the cylinder liner during operation of the engine. Particularly, a so-called "valve bridge portion" (a portion between an intake port and an exhaust port and a portion between these ports and a precombustion chamber hole or a fuel injection nozzle hole) cannot possess sufficient size and thickness due to its structure. Thus, the valve bridge portion is structurely weak and cracking may occur in the elements of the valve bridge portion.
A conventional measure to overcome the above-described problems is as follows: In casting the cylinder head, different metal is inserted to those portions which require high strength as disclosed in Japanese Utility Model Registration Application Second Publication No. 48-25923 and hardening of various degrees is applied to the intermediate product in accordance with thermal stress occurring in the final product during the operation of the engine as disclosed in Japanese Utility Model Registration Application Second Publication No. 63-8831.
However, recent engines have very high output and accordingly the thermal stress and mechanical load on the parts around the combustion chamber have increased greatly. Thus, reinforcement of those parts which are subjected to high thermal stress is not enough to eliminate the possibility of cracking.
On the other hand, a surface insulation treatment is applied to the parts around the combustion chamber in order to suppress thermal fatigue due to a temperature increase, as one aspect of improving the engine performance. One way of surface insulation is disclosed or instance in Japanese Utility Model Application Laid-Open No. 59-85348. In this application, a part of the cylinder head on the combustion chamber side is formed by ceramic material. This prevents cracking and improves thermal insulation properties.
FIGS. 17 and 18 show a cylinder head arrangement in line with the above proposal, in a plan view, and a sectional view, respectively. As illustrated, a recess portion 5 is formed at a valve bridge portion 4 between an intake port 1 and an exhaust port 2 of a cylinder head. A fuel injection nozzle installation hole 3 is bored in the valve bridge portion 4. The recess portion 5 is filled with ceramic material which forms a ceramic layer 6.
However, in a cylinder head made from cast iron, there is no adequate technique to join the cast iron material with the ceramic material. Therefore, the ceramic layer should be applied on the combustion chamber side cylinder head by bolts. The bolting cannot ensure a sufficient joint and consequently the ceramic part, which is a brittle part, may be broken due to vibrations during engine operation.
In another example, a ceramic port liner is inserted in the exhaust manifold in order to raise turbocharging efficiency by a thermal insulation of internal exhaust gas. In such a structure, the ceramic liner is cast as an insert as the exhaust manifold is cast. This raises the problem that the brittle ceramic part will be broken by a thermal expansion difference between the ceramic part and the cast iron part and stress produced upon solidification shrinkage. Even if the cracking does not appear during and after the casting operation, the parts may be broken by vibrations during the engine operation.
Another joint technique for the cast iron and the ceramic part has been proposed. An appropriate amount of metallic particles is mixed with ceramic particles and the mixture is sintered. Then, the sintered element is cast as an insert. According to this technique, the metallic particles are metallographically joined with the melt of cast iron. As a result, the ceramics and the cast iron are combined with each other very tightly.
The above proposal, however, has following drawbacks: First, if the ceramic particulates and the metallic particulates exist in a segregated state in the product, the thermal strength, the thermal insulation property and a deformation-resistance of the product are lowered and the durability of the product is shortened. In addition, it is very difficult to manufacture a product having the ceramic particulates and the metallic particulates distributed homogeneously. Very strict quality control is required to obtain a homogeneous product.